1. Field of the Invention
The present invention pertains to a novel method of producing certain substituted 2-benzoylbenzoic acid derivatives which are useful as intermediates for preparing valuable chromogenic phthalide compounds.
2. Prior Art
Several structurally diverse types of organic compounds have been found useful as color formers for carbonless duplicating systems and thermal marking systems. Among the more important of these compounds are the phthalides, for example the 3,3-diarylphthalides disclosed in U.S. Pat. Nos. Re. 23,024 and 4,096,176, the 3-aryl-3-heteroarylphthalides disclosed in U.S. Pat. Nos. 3,491,112 and 4,045,458, and the 3-aryl-3-(diarylamino)phthalides disclosed in U.S. Pat. No. 4,168,378. Of the several methods disclosed for preparing these compounds, the most versatile appears to be the reaction of a 2-benzoylbenzoic acid with an aniline derivative, an aromatic heterocyclic nitrogen compound or a diarylamine derivative to produce respectively a 3,3-diarylphthalide, a 3-aryl-3-heteroarylphthalide or a 3-aryl-3-(diarylamino)phthalide. Thus this method permits the preparation of a wide variety of phthalide color formers from a single intermediate in a one step reaction. Unfortunately however, when applied to the preparation of phthalides containing substituents in the phthalide ring, the apparent advantages of this process are significantly diminished because the requisite substituted 2-benzoylbenzoic acid intermediates, which are ordinarily obtained by reacting an appropriate substituted phthalic anhydride with an aniline derivative, are not readily accessible. The synthesis of substituted phthalic anhydrides is complex and unsuited to industrial production, and moreover, as would be expected, the reaction of a substituted phthalic anhydride with an aniline derivative produces a mixture of isomeric substituted 2-benzoylbenzoic acids. Thus there is need for a simple and economical method of producing pure substituted 2-benzoylbenzoic acids in high yield.
The following references appear to constitute the most pertinent prior art relative to the instant invention.
Doebner, Ann. 217, 257 (1883), discloses the reaction of malachite green with sulfuric acid to give 4-(dimethylamino)-benzophenone.
Wichelhaus, Chem. Ber. 19, 107-110 (1886), discloses the acid hydrolysis of crystal violet to give 4-4'-bis(dimethylamino)benzophenone (Michler's ketone). ##STR1##